The present invention relates to a plate heat exchanger. More particularly, the invention concerns a plate heat exchanger consisting of a stack of heat transfer plates provided with through inlet ports forming an inlet channel through the stack and, between the heat transfer plates, sealing means which together with the heat transfer plates in every other plate interspace delimit a first flow passage for one fluid and, in each remaining interspace, delimit a second flow passage for another fluid, wherein the inlet channel connects with each first flow passage by means of at least one inlet passage and is prevented from entry to the second flow passage(s) by sealing means which is located in a primary sealing area around each respective inlet port.
Typically, such plate heat exchangers have equally sized inlet and outlet ports for single phase heat exchange because the fluid density changes relatively little along the heat transfer channel connecting inlet to outlet ports.
In two phase heat transfer during evaporation or condensation, a transition from liquid to gas occurs resulting in very large density changes wherein the inlet and outlet velocities differ greatly. The different velocities produce different pressure drops along the through inlet ports with respect to the flow outlet ports from the stack of heat transfer plates.
Further, in the process of evaporation, dynamic boiling instabilities can result when produced vapor is able to displace liquid accumulated within an inlet port. This displaced liquid can enter the heat transfer passage and produce a transient over feed of the evaporating liquid. This transient over feed can produce control difficulties in many compact forms of heat exchangers where the liquid hold up capacity within the stack of plates is relatively small in relation to that of the inlet channel.
Additional problems of ensuring that each first flow passage is fed with an equal rate of evaporating fluid from the inlet port occurs when this fluid consists of two phases not one. Not only is it important to produce equal mass distribution but also to ensure equal phase distribution. This, however, is much more difficult to ensure yet, for evaporation, equal distribution of the inlet liquid phase to each first flow passage is crucial to ensure optimum heat transfer performance.
In a previous U.S. Pat. No. 3,735,793, a means of flow distribution down a flow port was conceived in which a large pressure drop was created, compared to along the inlet or outlet port, into each plate pair by means of providing each plate with a small diameter feed hole compared to the diameter of the inlet or outlet port. In this way, the feed rate into any plate pair is largely controlled mainly by the pressure loss through the small hole and not by the feed pressure minus passage and outlet port pressure loss.
Prior art as described in Swedish patent application 8702608-4 produces restrictive means into each plate pair from the inlet port using a ring or washer which is positioned around the feed port and through which flow is delimited by means of a cut, slot or drilled hole through this ring so producing a pressure drop creating orifice. These devices have not proven satisfactory for two main reasons. One, the high cost of production and two, the ring or washer is permanently bonded between the plate pair at the time of manufacture and has to be very accurately aligned so as not to interfere with the joining process which creates the plate pair.
The other major disadvantage is that the plate pair created completely encloses the pressure drop creating device which, once enclosed, can not be adjusted to compensate for changing process requirements, for example increased flow rates or changes in process fluids.
The present invention has been made from a consideration of the problems and disadvantages of the known heat exchangers aforementioned.
The purpose of this invention is to obtain a well defined pressure drop creating opening from the feed port into the plate passages.
A more preferred purpose of the invention is to provide a construction in which such pressure drop creating opening can be easily altered without changing the manufacturing process of the metal heat exchange plates.
The invention broadly consists in the heat transfer plates forming the first flow passage having a gasket arranged therebetween around the respective inlet ports.
Preferably, the gasket is not fully enclosed by the plates in the direction leading back into the inlet channel. In this way, the gasket remains accessible from this direction for removal or refit between a pair of plates even when the plates have been permanently joined together by a weld or braze in the primary sealing area.
Advantageously, the inlet passage is delimited by means of at least one of the heat transfer plates and/or by means for fluid to communicate directly through the gasket.
In one arrangement, the inlet passage is formed by a tube which extends through the gasket and provides pressure drop producing means connecting the inlet channel to the first flow passage. In this way, the pressure drop producing means can be adjusted by substituting different gasket/tube assemblies.
This design provides a much more flexible means to accommodate process changes without resorting to the re-manufacture of the expensive thermal plate pairs and, by standardizing the manufacture of the most costly components, namely the plates themselves, reduces overall manufacturing costs.
Previous art which uses a small orifice hole or pressed channel to effect a large pressure drop does not necessarily ensure equal phase distribution to all plate pairs along a feed port unless the feed is considered to act like a homogenous single phase.
The two phase feed to an evaporator will often not be homogenous but can be stratified due to a gas buoyancy effect with the result that plates will be fed with a different gas to liquid ratio at the entrance to the feed port than at the end of the feed port.
To facilitate correct gas distribution to each channel when the feed becomes stratified, the entrance to the pressure drop device must be a function of the plate pair position along the feed port. Each plate pair must thus differ from each other for optimum performance and can no longer be standardized provided the pressure drop means is made part of the thermal plates. The use of a separate component to provide the pressure drop means is beneficial for good operation.
Furthermore, the ability to access the pressure drop producing means externally has many advantages over that of designs employing a fixed means enclosed within the plate pair and produced by some plate feature i.e. hole or pressure detail. These advantages include:
1. If the pressure drop creating means should become blocked it may be easily accessed and cleaned.
2. If the pressure drop creating means becomes worn or damaged it can be simply replaced.
3. If the thermal duty changes then the pressure drop can be easily adjusted to suit by fitting a new gasket component.
4. The position and design of the pressure drop creating means can be varied with respect to position of any given plate pair within an operating plate stack so as to optimize two phase flow distribution.
The invention will now be described in detail in the following with references to the below drawings.